Many integrated circuits have two or more metal layers for interconnecting devices in the integrated circuit. FIG. 1 shows a cross-sectional view of a two metal layer interconnect structure 100 having a metal-1 layer 110, a metal-2 layer 120, and a via 130 for interconnecting metal-1 110 and metal-2 layer 120. Via 130 has a barrier layer 132 separating metal-1 layer 110 and metal-2 layer 120. Barrier layer 132 is formed by a layer 134 and a layer 136, which in this example, are made of TiN for adhesion of Metal-2 layer 120 and anti-reflective coating (ARC) for Metal-1 layer 110, respectively. Metal-1 and Metal-2 layers 110 and 120 are made of Al or Al alloy.
One of the failure modes of structure 100 is caused by electromigration. Electromigration refers to the physical displacement or scattering of atoms of the conducting medium (e.g., the Al of the metal layers) arising from collisions with electrons moving in the opposite direction of the current. As electromigration occurs, the resistance of the affected metal layer can increase to the point of failure.
Also, barrier layer 132 can be a problem because barrier layer 132 causes via 130 to be more resistive than a purely metal via. Consequently, the speed at which signals propagate through the metal/barrier/metal structure is decreased. The resistance of barrier layer 132 increases with thickness. However, the lifetime of the integrated circuit also increases with barrier thickness (discussed below in conjunction with FIG. 2). Thus, the designer can trade off speed for lifetime by varying the barrier layer thickness.
FIG. 2 illustrates the effects of electromigration near barrier layer 132. A current I.sub.1 is conducted from metal-2 layer 120 to metal-1 layer 110 though barrier layer 132. Thus, electrons are moving in the direction opposite of the current; i.e., from metal-1 layer 110 to metal-2 layer 120.
The current density is generally greatest along the "line of least resistance", represented by arrow 200. Thus, the greatest density of the "electron wind" is generally along arrow 200 in the opposite direction. Due to electromigration, Al atoms in metal-2 layer 120 tend to be scattered from areas of metal-2 layer 120 having high current density. As a result, voids 210 in metal-2 layer 120 are created in areas along arrow 200. Consequently, the area (perpendicular to current flow) of metal-2 layer 120 is decreased, which increases the resistance of metal-2 layer 120. As more electromigration occurs, more voids are created causing the resistance of metal-2 layer 120 to increase to a point where the device is considered to have failed.
FIG. 2 also illustrates the conventional method for determining the lifetime of such structures. Current is conducted in one direction until the resistance reaches a predetermined failure criterion. In this example, the current flows from metal-2 layer 120 to metal-1 layer 110. This method measures the lifetime of the structure, but cannot determine the effects of the barrier on the lifetime. For example, because another factor may be the limiting factor on lifetime, varying the barrier's thickness may only change the observed lifetime a relatively small amount when using the conventional method.